Every amp that comes into the shop gets a thorough diagnostic checkout and comprehensive inspection for existing and potential problems. Defective or substandard components are replaced with new components of equal or superior quality. Critical performance and tone affecting parameters are measured to make sure your amp meets design specs.  These include all power supply voltages, plate voltages, grid and screen current draw and grid bias.  If tubes are replaced, they will be selected for their sonic attributes, ability to adequately handle your amp's supplied plate voltage and closely matched (in the case of output tubes). All tube sockets are cleaned and tightened, pots cleaned and all electronic components are checked visually for signs of stress or heat damage. The final analysis, of course, is listening to the amp to ensure it produces the characteristic tone it was designed to.